Differentiation of agonist conformation and antagonist conformation in multiple opioid receptors

To differentiate the opiate (naloxone) receptor and the enkephalin receptor in rat brain, we solubilized the receptor molecules by detergent and determined the molecular weights by gel filtration. The receptor preparation was bound to [ 3H]naloxone or [ 3H]Met 5-enkephalin, and was solubilized by Triton X-100. On gel chromatography with a Sepharose 6B column, the agonist and the antagonist conformation of opioid receptors eluted as molecules with the molecular weights of 240,000 and 120,000, and with Stokes' radii of 5.5 nm and 4.3 nm, respectively. Further, it was also disclosed that Na + was bound to the antagonist conformation of opioid receptors but not to the agonist conformation.